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<!--
SPDX-FileCopyrightText: 2023 The "Notes on OpenPGP" project
SPDX-License-Identifier: CC-BY-SA-4.0
-->
(zoom_certificates)=
# Zooming in: Packet structure of certificates
Now that we've established the concepts and components that make up OpenPGP certificates, let's look at the internal details of an example certificate.
## A very minimal OpenPGP certificate
In this section, we will examine a very minimal version of a "public key" variant of [Alice's OpenPGP key](alice_priv), specifically an OpenPGP certificate that excludes private key material.
To achieve this, we will use the Sequoia-PGP tool `sq` to handle and transform our example OpenPGP key, as well as to inspect internal OpenPGP packet data.
Starting from [Alice's OpenPGP private key](alice_priv), we first produce the corresponding public key/certificate using the following command:
```text
$ sq key extract-cert alice.priv > alice.pub
```
(split_alice)=
### Splitting the OpenPGP certificate into packets
To create a very minimal version of Alice's certificate, we will split the data in `alice.pub` into its component packets and reassemble only the relevant ones back into a new variant.
Execute the following command to achieve this:
```text
$ sq packet split alice.pub
```
With this command, `sq` generates a set of files, each containing an individual OpenPGP packet extracted from the original full certificate in `alice.pub`:
```text
alice.pub-0--PublicKey
alice.pub-1--Signature
alice.pub-2--UserID
alice.pub-3--Signature
alice.pub-4--PublicSubkey
alice.pub-5--Signature
alice.pub-6--PublicSubkey
alice.pub-7--Signature
alice.pub-8--PublicSubkey
alice.pub-9--Signature
```
```{figure} diag/certificate_packet_list.png
:name: fig-certificate-packet-list
:alt: Depicts a box with white background and the title "Certificate packet list". Inside, a list of several boxes on white background and varying frame colors represent a list of OpenPGP packets from top to bottom. The first box, with green frame, represents the "Public-Key packet", and includes the green public key symbol. The second box, with yellow frame, represents a "Signature packet" ("Direct Key Signature") and includes the green cryptographic signature symbol. The third box, with black frame, represents a "User ID packet", and includes the black User ID symbol. The fourth box, with yellow frame, represents a "Signature packet" ("Certifying self-signature for User ID"), and includes the green cryptographic signature symbol. The fifth box, with green frame, represents a "Public-Subkey packet" and includes the green public key symbol. The sixth box, with yellow frame, represents a "Signature packet" ("Subkey binding signature") and includes the green cryptographic signature symbol. The seventh box, with green frame, represents a "Public-Subkey packet" and includes the green public key symbol. The eighth box, with yellow frame, represents a "Signature packet" ("Subkey binding signature") and includes the green cryptographic signature symbol. The ninth box, with green frame, represents a "Public-Subkey packet" and includes the green public key symbol. The tenth box, with yellow frame, represents a "Signature packet" ("Subkey binding signature") and includes the green cryptographic signature symbol.
Overview of the packets in Alice's OpenPGP certificate
```
This process allows us to focus on the specific packets within Alice's OpenPGP certificate.
### Assembling packets into an OpenPGP certificate
In this step, we'll merge the first two packets of Alice's certificate to create a very minimal certificate:
Execute the following:
```text
$ sq packet join alice.pub-0--PublicKey alice.pub-1--Signature --output alice_minimal.pub
```
This command combines the contents of `alice.pub-0--PublicKey` and `alice.pub-1--Signature` into a single file named `alice_minimal.pub`.
### Inspecting this certificate
This version of Alice's certificate contains just two packets:
- the [*Public-Key packet*](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-10.html#name-public-key-packet-formats) for the primary key, and
- a [*Direct Key Signature*](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-10.html#sigtype-direct-key), which is a self-signature that binds metadata to the primary key.
This is the shape of the packets we'll explore in the subsequent sections:
```{figure} diag/pubcert-minimal.png
:name: fig-public-certificate-minimal
:alt: TODO
:width: 40%
A minimal OpenPGP certificate, visualized
```
```{admonition} VISUAL
:class: warning
This diagram needs adjustments about
- what exactly is signed
- fix naming of fields?
We could show repeat-copies of the individual packet visualization again, below for each packet-related section.
```
In real-world scenarios, OpenPGP certificates are typically far more complex than this minimal example. However, this is indeed a valid OpenPGP certificate. In the following sections, we will introduce more components to this certificate, increasing its complexity and exploring their details.
In ASCII-armored representation, this very minimal key appears as follows:
```text
-----BEGIN PGP PUBLIC KEY BLOCK-----
xioGZRbqphsAAAAgUyTpQ6+rFfdu1bUSmHlpzRtdEGXr50Liq0f0hrOuZT7CtgYf
GwoAAAA9BYJlFuqmBYkFpI+9AwsJBwMVCggCmwECHgEiIQaqoYy7JUaFxYNYMgVj
/Te2fzMA+fsOxFc3jNKfECaYswAAAAoJEKqhjLslRoXFZ0cgouNjgeNr0E9W18g4
gAIl6FM5SWuQxg12j0S07ExCOI5NPRDCrSnAV85mAXOzeIGeiVLPQ40oEal3CX/L
+BXIoY2sIEQrLd4TAEEy0BA8aQZTPEmMdiOCM1QB+V+BQZAO
=5nyq
-----END PGP PUBLIC KEY BLOCK-----
```
The output of `sq` is presented as a block of text. We will now decode this OpenPGP data and inspect the two packets it contains.
To achieve this, we will use the Sequoia-PGP tool `sq` and run the `packet dump` subcommand:
```text
$ sq packet dump --hex alice_minimal.pub
```
This will allow us to gain a detailed understanding of the packet contents.
(public_key)=
### Public-Key packet
The output begins with a (primary) [Public-Key packet](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-10.html#name-public-key-packet-formats):
```text
Public-Key Packet, new CTB, 2 header bytes + 42 bytes
Version: 6
Creation time: 2023-09-29 15:17:58 UTC
Pk algo: Ed25519
Pk size: 256 bits
Fingerprint: AAA18CBB254685C58358320563FD37B67F3300F9FB0EC457378CD29F102698B3
KeyID: AAA18CBB254685C5
00000000 c6 CTB
00000001 2a length
00000002 06 version
00000003 65 16 ea a6 creation_time
00000007 1b pk_algo
00000008 00 00 00 20 public_len
0000000c 53 24 e9 43 ed25519_public
00000010 af ab 15 f7 6e d5 b5 12 98 79 69 cd 1b 5d 10 65
00000020 eb e7 42 e2 ab 47 f4 86 b3 ae 65 3e
```
The Public-Key packet consists primarily of the cryptographic key data. Let's look at the packet field by field:
**OpenPGP packet syntax**
The first fields of a packet are governed by the general [Packet Syntax](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-10.html#name-packet-syntax):
- `CTB: 0xc6`[^CTB]: This is the [packet type ID](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-10.html#name-packet-headers) for this packet. The binary representation of the value `0xc6` is `11000110`. The first two bits show that the packet is in *OpenPGP packet format* (as opposed to in *Legacy packet format*) and the remaining 6 bits encode the type ID value, which is "6." This type ID value corresponds to a Public-Key packet, as listed in the [packet type IDs](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-10.html#name-packet-tags).
- `length: 0x2a`: This indicates the remaining length of this packet.
**Public-Key packet syntax**
The packet type ID ("6") defines the semantics of the following data within the packet. In this case, it is a Public-Key packet, which is a kind of [Key Material Packet](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-10.html#name-key-material-packets).
- `version: 0x06`: The key material is in version 6 format. This means that the next part of the packet adheres to the structure of [Version 6 Public Keys](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-10.html#name-version-6-public-keys).
- `creation_time: 0x6516eaa6`: This field represents the key's creation time. (See also [Time Fields](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-10.html#name-time-fields)).
- `pk_algo: 0x1b`: This corresponds to the key's public-key algorithm ID, which has a decimal value of 27. Refer to the list of [Public-Key Algorithms](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-10.html#name-public-key-algorithms)) for more details.
- `public_len: 0x00000020`: This field specifies the octet count for the subsequent public key material. In this case, it represents the length of the following `ed25519_public` field.
- `ed25519_public`: This is the [algorithm-specific representation](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-10.html#name-algorithm-specific-part-for-ed2) of the public key material. The format is based on the value of `pk_algo`, which, in this case, is 32 bytes of Ed25519 public key data.
[^CTB]: Sequoia uses the term CTB (Cipher Type Byte) to refer to the RFC's [packet type ID](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-10.html#name-packet-headers). In earlier RFC versions, this field was known as the "Packet Tag."
Note that the *Public-Key packet* contains only the public part of the key.
```{figure} diag/public-key_packet.png
:name: fig-public-key-packet
:alt: Depicts a box with white background and title "Public-Key packet". In the center a box with white background and green frame is shown. Inside it several items are listed, separated by green dotted horizontal lines. The first three are "Version", "Creation Time", "Public-Key Algorithm" written in black. The last one is written in green and reads "Public Key Material" and has the green public key symbol at its right side.
Structure of a Public-Key packet.
```
(zooming_in_dks)=
### Direct Key Signature
The next packet in the certificate is a [*Direct Key Signature*](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-10.html#sigtype-direct-key), which plays a crucial role in binding specific information to the primary key. This signature is contained within the file `alice.pub-1--Signature`.
This packet binds the data within the signature subpackets with the primary key. Each entry under "Signature Packet -> Hashed area" is one signature subpacket, providing essential information such as algorithm preferences, including *symmetric algorithm preference* and *hash algorithm preferences*.
```text
Signature Packet, new CTB, 2 header bytes + 182 bytes
Version: 6
Type: DirectKey
Pk algo: Ed25519
Hash algo: SHA512
Hashed area:
Signature creation time: 2023-09-29 15:17:58 UTC (critical)
Key expiration time: P1095DT62781S (critical)
Symmetric algo preferences: AES256, AES128
Hash preferences: SHA512, SHA256
Key flags: C (critical)
Features: MDC
Issuer Fingerprint: AAA18CBB254685C58358320563FD37B67F3300F9FB0EC457378CD29F102698B3
Unhashed area:
Issuer: AAA18CBB254685C5
Digest prefix: 6747
Level: 0 (signature over data)
00000000 c2 CTB
00000001 b6 length
00000002 06 version
00000003 1f type
00000004 1b pk_algo
00000005 0a hash_algo
00000006 00 00 00 3d hashed_area_len
0000000a 05 subpacket length
0000000b 82 subpacket tag
0000000c 65 16 ea a6 sig creation time
00000010 05 subpacket length
00000011 89 subpacket tag
00000012 05 a4 8f bd key expiry time
00000016 03 subpacket length
00000017 0b subpacket tag
00000018 09 07 pref sym algos
0000001a 03 subpacket length
0000001b 15 subpacket tag
0000001c 0a 08 pref hash algos
0000001e 02 subpacket length
0000001f 9b subpacket tag
00000020 01 key flags
00000021 02 subpacket length
00000022 1e subpacket tag
00000023 01 features
00000024 22 subpacket length
00000025 21 subpacket tag
00000026 06 version
00000027 aa a1 8c bb 25 46 85 c5 83 issuer fp
00000030 58 32 05 63 fd 37 b6 7f 33 00 f9 fb 0e c4 57 37
00000040 8c d2 9f 10 26 98 b3
00000047 00 00 00 0a unhashed_area_len
0000004b 09 subpacket length
0000004c 10 subpacket tag
0000004d aa a1 8c issuer
00000050 bb 25 46 85 c5
00000055 67 digest_prefix1
00000056 47 digest_prefix2
00000057 20 salt_len
00000058 a2 e3 63 81 e3 6b d0 4f salt
00000060 56 d7 c8 38 80 02 25 e8 53 39 49 6b 90 c6 0d 76
00000070 8f 44 b4 ec 4c 42 38 8e
00000078 4d 3d 10 c2 ad 29 c0 57 ed25519_sig
00000080 ce 66 01 73 b3 78 81 9e 89 52 cf 43 8d 28 11 a9
00000090 77 09 7f cb f8 15 c8 a1 8d ac 20 44 2b 2d de 13
000000a0 00 41 32 d0 10 3c 69 06 53 3c 49 8c 76 23 82 33
000000b0 54 01 f9 5f 81 41 90 0e
```
Below is a field-by-field examination of the packet:
**OpenPGP packet syntax**
The first fields of a packet are governed by the general [Packet Syntax](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-10.html#name-packet-syntax):
- `CTB: 0xc2`: This field indicates the Packet type ID for this packet. Bits 7 and 6 show that the packet is in “OpenPGP packet format.” The remaining 6 bits encode the type ID’s value, which is “2” for a Signature packet.
- `length: 0xb6`: This field shows the remaining length of this packet.
**Signature packet syntax**
The packet type ID (“2”) defines the semantics of the remaining data in the packet. In this case, as it indicates a [Signature packet](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-10.html#signature-packet), the following data is specific to this packet type:
- `version: 0x06`: This is a version 6 signature.
- `type: 0x1f`: This indicates the [Signature Type](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-10.html#name-signature-types).
- `pk_algo: 0x1b`: This specifies the Public-Key algorithm ID, with decimal 27 corresponding to [Ed25519](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-10.html#name-public-key-algorithms)).
- `hash_algo: 0x0a`: This specifies the hash algorithm ID, with decimal 10 corresponding to [SHA2-512](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-10.html#name-hash-algorithms)).
- `hashed_area_len: 0x0000003d`: This specifies the length of the following hashed subpacket data.
The next segment of this packet contains the hashed subpacket data.
In OpenPGP Signatures, there are two sets of subpacket data: hashed and unhashed. Hashed subpackets are protected by the digital signature of the packet, while unhashed subpackets are not.
A subpacket data set in an OpenPGP Signature contains a list of zero or more Signature subpackets.
The following subpacket data consists of sets of "subpacket length, subpacket type ID, data." Each subpacket is displayed as one line, starting with the [subpacket type description](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-10.html#name-signature-subpacket-specifi) (based on the subpacket type ID). Note that bit 7 of the subpacket type ID signals if that subpacket is ["critical."](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-10.html#section-5.2.3.7-10)
```{note}
Critical here means that the receiver must interpret the subpacket and is expected to fail, otherwise. Non-critical subpackets may be ignored by the receiver.
```
The subpacket details are as follows:
- [**Signature Creation Time**](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-10.html#signature-creation-subpacket)
- Type: `2`
- Critical: `Yes`
- Value: `0x6516eaa6`
- Notes: See also [Time Fields](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-10.html#name-time-fields).
- [**Key Expiration Time**](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-10.html#key-expiration-subpacket)
- Type: `9`
- Critical: `Yes`
- Value: `0x05a48fbd`
- Notes: Defined as number of seconds after the key creation time
- [**Preferred Symmetric Ciphers for v1 SEIPD**](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-10.html#preferred-v1-seipd)
- Type: `11`
- Critical: `No`
- Value: `0x09 0x07`
- Notes: Values correspond to *AES with 256-bit key* and *AES with 128-bit key*
- [**Preferred Hash Algorithms**](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-10.html#preferred-hashes-subpacket)
- Type: `21`
- Critical: `No`
- Value: `0x0a 0x08`
- Notes: Values correspond to *SHA2-512* and *SHA2-256*.
- [**Key Flags**](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-10.html#key-flags)
- Type: `27`
- Critical: `Yes`
- Value: `0x01`
- Notes: Value corresponds to the *certifications* key flag.
- [**Features**](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-10.html#features-subpacket)
- Type: `30`
- Critical: `No`
- Value: `0x01`
- Notes: Value corresponds to *Symmetrically Encrypted Integrity Protected Data packet version 1*
- [**Issuer Fingerprint**](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-10.html#issuer-fingerprint-subpacket)
- Type: `33`
- Critical: `No`
- Value: `aaa18cbb254685c58358320563fd37b67f3300f9fb0ec457378cd29f102698b3`
- Notes: The fingerprint identifies the component key that issued the signature in this packet. In this instance, the value is the primary key fingerprint of the certificate we're looking at.
The next part of this packet contains unhashed subpacket data:
- `unhashed_area_len: 0x0000000a`: length of the following unhashed subpacket data (value: 10 bytes).
As above, the following subpacket data consists of sets of subpacket length, subpacket type id, and data. In this case, only one subpacket follows:
- [**Issuer Key ID**](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-10.html#issuer-keyid-subpacket)
- Type: `16`
- Critical: `No`
- Value: `aaa18cbb254685c5`
- Notes: This is the shortened version 6 *Key ID* of the fingerprint of this certificate's primary key.
This concludes the unhashed subpacket data.
This next section shows the remaining fields of this signature packet, which relate to the cryptographic digital signature:
- `digest_prefix: 0x6747`: the left 16 bits of the signed hash digest
- `salt_len, salt`: a random [salt value](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-10.html#name-advantages-of-salted-signat) with size [matching the hash algorithm](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-10.html#hash-algorithms-registry))
- `ed25519_sig`: [algorithm-specific](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-10.html#name-algorithm-specific-fields-for-ed2) representation of the signature (here: 64 bytes of Ed25519 signature)
The hash digest is calculated from the following data (see [Computing Signatures](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-11.html#name-computing-signatures) in the RFC):
- the signature's salt
- the serialized primary key's public data
- the serialized direct key signature packet (excluding the unhashed area)
The signature is calculated from this hash digest.
```{figure} diag/direct_key_signature_packet.png
:name: fig-direct-key-signature-packet
:alt: Depicts a box with white background, title "Signature packet" and subtitle "Direct Key Signature (type ID 0x1F)". In the center a box with white background and yellow frame is shown. Inside it several items are listed, separated by yellow dotted horizontal lines. The first three are "Version", "Public-Key Algorithm" and "Hash Algorithm". The fourth item is called "Hashed area" and confines further sub-items by a light-yellow frame on the top and left side. The sub-items are "Signature Creation Time", "Key Expiration Time", "Preferred Symmetric Ciphers for v1 SEIPD", "Preferred Hash Algorithms", "Key Flags", "Features" and "Issuer Fingerprint". The fifth item is named "Unhashed area" and again introduces an area for sub-items, this time using a light-gray border on the top and left side. The unhashed area has no sub-items though. The last item is called "Cryptographic Signature", with the subtitle "by the primary key over primary key, subkey and signature metadata" and includes the green cryptographic signature symbol on the right side.
Structure of a direct key signature packet.
```
(zoom_enc_subkey)=
## Encryption subkey
Let's now look at a subkey in Alice's OpenPGP certificate. A subkey, when linked to an OpenPGP certificate via its primary key, consists of two elements:
- a key packet that contains the component key itself, and
- a signature packet that links this component key to the primary key and, implicitly, to the full OpenPGP certificate.
We will use the files containing individual packets of Alice's certificate, which we separated above. In this split representation, the encryption subkey is stored in `alice.pub-4--PublicSubkey`, while the associated binding self-signature is stored in `alice.pub-5--Signature`.
````{note}
It's common to look at a packet dump for a full OpenPGP certificate as shown below:
```text
$ sq packet dump --hex alice.pub
```
This command shows the details for the full series of packets in an OpenPGP certificate (refer to the list of [packets of Alice's certificate](split_alice)). Finding a particular packet in that list can take a bit of focus and practice though.
In the following sections,we make it easier for ourselves by directly examining individual packets from the files we created with `sq packet split` above.
````
### Public-Subkey packet
We'll now look at the *Public-Subkey packet* that contains the component key data of this subkey:
```text
$ sq packet dump --hex alice.pub-4--PublicSubkey
Public-Subkey Packet, new CTB, 2 header bytes + 42 bytes
Version: 6
Creation time: 2023-09-29 15:17:58 UTC
Pk algo: X25519
Pk size: 256 bits
Fingerprint: C0A58384A438E5A14F73712426A4D45DBAEEF4A39E6B30B09D5513F978ACCA94
KeyID: C0A58384A438E5A1
00000000 ce CTB
00000001 2a length
00000002 06 version
00000003 65 16 ea a6 creation_time
00000007 19 pk_algo
00000008 00 00 00 20 public_len
0000000c d1 ae 87 d7 x25519_public
00000010 cc 42 af 99 34 c5 c2 5c ca fa b7 4a c8 43 fc 86
00000020 35 2a 46 01 f3 cc 00 f5 4a 09 3e 3f
```
Notice that the structure of this *Public-Subkey packet* mirrors the primary key's [*Public-Key packet*](public_key) above. However, there are notable differences between the two packets:
- The packet type ID (`CTB`) in this packet shows type 14 ([*Public-Subkey packet*](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-10.html#name-public-subkey-packet-tag-14)).
- The `pk_algo` value is set to `0x19` (decimal 25), which [corresponds to X25519](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-10.html#name-public-key-algorithms). Notably, though both the primary key and this subkey use a cryptographic mechanism based on Curve25519, the encryption key uses Curve 25519 in a different way: namely, X25519 is a Diffie–Hellman function constructed from Curve25519.
- Accordingly, the public part of the cryptographic key pair is labeled `x25519_public`, as implied by the value (`0x19`) of `pk_algo`. However, the actual data is just 32 bytes of cryptographic key material, without any type information.
### Subkey binding signature
The aforementioned subkey packet is disconnected from the OpenPGP certificate to which it belongs. The link between the subkey and the complete OpenPGP certificate is made with a cryptographic signature, generated by primary key of the OpenPGP certificate.
The type of signature is called a *subkey binding signature*, because it "binds" or connects the subkey to the rest of the key.
```{admonition} VISUAL
:class: warning
Add detailed packet diagram analogous to 4.6.1
```
```{admonition} TODO
:class: warning
david points out: "The information on metadata in binding signatures may also make sense in other contexts (direct key signature)?"
Should this text go elsewhere?
- 4.2.3?
- ch 6?
```
The signature does more than just bind the subkey; it also carries additional metadata about the subkey. This metadata is in the binding signature, and not in the subkey packet, because it may change over time, while the subkey packet itself remains unchanged. This evolving metadata is stored in self-signatures: if the key holder wants to modify the metadata (for example, to change the key's expiration time), a newer version of the same signature type can be issued. The recipient OpenPGP software will recognize that the newer self-signature supersedes the older one, and that the metadata in the newer signature reflects the most current intent of the key holder.
Note that this subkey binding signature packet is quite similar to the Direct Key Signature discussed above. Both signatures serve a similar purpose in adding metadata to a component key, particularly as the hashed subpacket data contains much of the same metadata elements.
```text
$ sq packet dump --hex alice.pub-5--Signature
Signature Packet, new CTB, 2 header bytes + 171 bytes
Version: 6
Type: SubkeyBinding
Pk algo: Ed25519
Hash algo: SHA512
Hashed area:
Signature creation time: 2023-09-29 15:17:58 UTC (critical)
Key expiration time: P1095DT62781S (critical)
Key flags: EtEr (critical)
Issuer Fingerprint: AAA18CBB254685C58358320563FD37B67F3300F9FB0EC457378CD29F102698B3
Unhashed area:
Issuer: AAA18CBB254685C5
Digest prefix: 2289
Level: 0 (signature over data)
00000000 c2 CTB
00000001 ab length
00000002 06 version
00000003 18 type
00000004 1b pk_algo
00000005 0a hash_algo
00000006 00 00 00 32 hashed_area_len
0000000a 05 subpacket length
0000000b 82 subpacket tag
0000000c 65 16 ea a6 sig creation time
00000010 05 subpacket length
00000011 89 subpacket tag
00000012 05 a4 8f bd key expiry time
00000016 02 subpacket length
00000017 9b subpacket tag
00000018 0c key flags
00000019 22 subpacket length
0000001a 21 subpacket tag
0000001b 06 version
0000001c aa a1 8c bb issuer fp
00000020 25 46 85 c5 83 58 32 05 63 fd 37 b6 7f 33 00 f9
00000030 fb 0e c4 57 37 8c d2 9f 10 26 98 b3
0000003c 00 00 00 0a unhashed_area_len
00000040 09 subpacket length
00000041 10 subpacket tag
00000042 aa a1 8c bb 25 46 85 c5 issuer
0000004a 22 digest_prefix1
0000004b 89 digest_prefix2
0000004c 20 salt_len
0000004d 0b 0c 89 salt
00000050 b5 ab 15 e3 7f e4 4d b9 a7 ef 71 48 14 3b ab 26
00000060 5f 34 7f 6d 48 2e 9f 78 48 58 6d 9a fb
0000006d 6d b2 db ed25519_sig
00000070 2f 97 8e c8 12 fc 57 7f 85 aa d1 59 bc 80 40 0b
00000080 be 2e f0 e1 23 2d bf 4b 71 7e d0 e4 c0 36 e4 d2
00000090 cf b2 9f b4 a8 4f 3e 2a 21 89 74 c2 33 55 af ac
000000a0 41 36 1b 2b 60 09 f2 d9 19 f4 41 12 0b
```
The analysis of this packet dump will be less extensive, given that its structure mirrors the *Direct Key Signature* explored above.
One notable difference is the `type` field, showing that this signature is of type `0x18` ([Subkey Binding Signature](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-10.html#name-subkey-binding-signature-si)).
The `pk_algo` value of this signature derives from the algorithm of the primary key (`0x1b`, corresponding to Ed25519). This signature is issued by the primary key, thus using the signing algorithm of the primary key. (The algorithm used to produce the cryptographic signature in this packet is entirely independent of the `pk_algo` of the key material of this subkey itself, which uses the X25519 mechanism.)
As shown in the text at the top of this packet dump, the hashed subpacket data contains four pieces of information:
- signature creation time: `2023-09-29 15:17:58 UTC` (**critical**)
- key expiration time: `P1095DT62781S` (**critical**)
- key flags: `EtEr` (**critical**) (encryption for communication, encryption for storage)
- issuer fingerprint: `AAA18CBB254685C58358320563FD37B67F3300F9FB0EC457378CD29F102698B3`
The rest of the packet mirrors the *Direct Key Signature* discussed above:
- a 16-bit digest prefix
- a salt value
- the cryptographic signature itself
The signature is calculated over a hash digest. In this case, the hash digest is derived from the following data:
- the signature's salt
- the serialized primary key's public data
- the serialized subkey's public data
- the serialized subkey binding signature packet (excluding the unhashed area)
Refer to [Computing Signatures](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-11.html#name-computing-signatures) in the RFC for details.
## Signing subkey
```{admonition} TODO
:class: warning
write
```
```text
$ sq packet dump --hex alice.pub-6--PublicSubkey
Public-Subkey Packet, new CTB, 2 header bytes + 42 bytes
Version: 6
Creation time: 2023-09-29 15:17:58 UTC
Pk algo: Ed25519
Pk size: 256 bits
Fingerprint: D07B24EC91A14DD240AC2D53E6C8A9E054949A41222EA738576ED19CAEA3DC99
KeyID: D07B24EC91A14DD2
00000000 ce CTB
00000001 2a length
00000002 06 version
00000003 65 16 ea a6 creation_time
00000007 1b pk_algo
00000008 00 00 00 20 public_len
0000000c 33 8c d4 f5 ed25519_public
00000010 1a 73 39 ef ce d6 0f 21 8d a0 58 a2 3c 3d 44 a8
00000020 59 e9 13 1f 12 9c 6f 19 d0 3d 40 a0
```
```text
$ sq packet dump --hex alice.pub-7--Signature
Signature Packet, new CTB, 3 header bytes + 325 bytes
Version: 6
Type: SubkeyBinding
Pk algo: Ed25519
Hash algo: SHA512
Hashed area:
Signature creation time: 2023-09-29 15:17:58 UTC (critical)
Key expiration time: P1095DT62781S (critical)
Key flags: S (critical)
Embedded signature: (critical)
Signature Packet
Version: 6
Type: PrimaryKeyBinding
Pk algo: Ed25519
Hash algo: SHA512
Hashed area:
Signature creation time: 2023-09-29 15:17:58 UTC (critical)
Issuer Fingerprint: D07B24EC91A14DD240AC2D53E6C8A9E054949A41222EA738576ED19CAEA3DC99
Digest prefix: 5365
Level: 0 (signature over data)
Issuer Fingerprint: AAA18CBB254685C58358320563FD37B67F3300F9FB0EC457378CD29F102698B3
Unhashed area:
Issuer: AAA18CBB254685C5
Digest prefix: 841C
Level: 0 (signature over data)
00000000 c2 CTB
00000001 c0 85 length
00000003 06 version
00000004 18 type
00000005 1b pk_algo
00000006 0a hash_algo
00000007 00 00 00 cc hashed_area_len
0000000b 05 subpacket length
0000000c 82 subpacket tag
0000000d 65 16 ea sig creation time
00000010 a6
00000011 05 subpacket length
00000012 89 subpacket tag
00000013 05 a4 8f bd key expiry time
00000017 02 subpacket length
00000018 9b subpacket tag
00000019 02 key flags
0000001a 99 subpacket length
0000001b a0 subpacket tag
0000001c 06 19 1b 0a embedded sig
00000020 00 00 00 29 05 82 65 16 ea a6 22 21 06 d0 7b 24
00000030 ec 91 a1 4d d2 40 ac 2d 53 e6 c8 a9 e0 54 94 9a
00000040 41 22 2e a7 38 57 6e d1 9c ae a3 dc 99 00 00 00
00000050 00 53 65 20 42 03 ad 0c db fc b5 9a 98 a6 15 27
00000060 e4 11 5e f5 f2 a0 3d bc ed 8d 94 27 41 09 f6 3c
00000070 4b f8 8a e5 af 73 e1 7d 54 07 40 3f f3 29 34 c2
00000080 e7 60 56 a5 e1 43 cb 08 ba 66 fe 8b 26 ce e7 cb
00000090 a5 3a 46 bb a5 c8 5d e4 6a de ae 49 e1 3e 07 bf
000000a0 c4 9e 98 14 2f 3e c5 f7 01 3e 3e 4f f6 18 2a ac
000000b0 bd ed 52 0c
000000b4 22 subpacket length
000000b5 21 subpacket tag
000000b6 06 version
000000b7 aa a1 8c bb 25 46 85 c5 83 issuer fp
000000c0 58 32 05 63 fd 37 b6 7f 33 00 f9 fb 0e c4 57 37
000000d0 8c d2 9f 10 26 98 b3
000000d7 00 00 00 0a unhashed_area_len
000000db 09 subpacket length
000000dc 10 subpacket tag
000000dd aa a1 8c issuer
000000e0 bb 25 46 85 c5
000000e5 84 digest_prefix1
000000e6 1c digest_prefix2
000000e7 20 salt_len
000000e8 23 3d b2 49 f3 02 4b 08 salt
000000f0 93 af ba 08 89 f0 e0 91 0f ab 22 26 aa b3 56 57
00000100 30 ea 95 29 06 60 6f 00
00000108 be 44 a1 95 38 a9 6b 3a ed25519_sig
00000110 3e 51 f0 55 09 b1 e2 91 a9 17 86 fa f5 1e 3f d0
00000120 28 46 3c ce 6e 88 14 37 32 ec 3d fa c6 01 ca e5
00000130 a9 4b b7 63 94 c3 0d 92 ab dc fa 23 50 71 60 31
00000140 a6 73 c8 33 5a 9c d9 0a
```
(zooming_in_user_id)=
## Adding an identity component
In this section, we'll look at an identity associated with Alice's certificate.
User IDs are a mechanism for connecting [identities](identity_components) with an OpenPGP certificate. Typically, a User ID is a string combining a name and an email address.
To understand the internal packet structure of this identity and its connection to the OpenPGP certificate, we'll examine two packets that constitute the identity component. One is the [User ID packet](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-10.html#name-user-id-packet-tag-13), located in the file `alice.pub-2--UserID`, which contains identity information. The other is a certifying self-signature, specifically a [Positive certification of a User ID and Public-Key packet](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-10.html#name-positive-certification-of-a) located in the file `alice.pub-3--Signature`. This certification, issued after substantial verification of the identity claim, validates the association between the User ID and the certificate's public key. These packets are snippets from Alice's full OpenPGP certificate.
### User ID packet
First, let's look at the User ID packet, which encodes an identity that is associated with an OpenPGP certificate:
```text
$ sq packet dump --hex alice.pub-2--UserID
User ID Packet, new CTB, 2 header bytes + 19 bytes
Value: <alice@example.org>
00000000 cd CTB
00000001 13 length
00000002 3c 61 6c 69 63 65 40 65 78 61 6d 70 6c 65 value
00000010 2e 6f 72 67 3e
```
- `CTB: 0xcd`: This is the packet type ID for this packet. Bits 7 and 6 show that the packet is in “OpenPGP packet format” (not “Legacy packet format”). The remaining 6 bits encode the type ID’s value: “13,” which is the value for a [User ID packet](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-10.html#name-user-id-packet-tag-13).
- `length: 0x13`: This field shows the remaining length of the packet (here: 19 bytes).
- `value`: This comprises 19 bytes of data that contain UTF-8 encoded text. The value corresponds to the string `<alice@example.org>`. With this identity component, Alice asserts usage and control over the specified email address. Note that the email address is enclosed in `<` and `>` characters, in line with the conventions of [RFC 2822](https://www.rfc-editor.org/rfc/rfc2822).
Essentially, a User ID packet is just a string marked as a User ID by the packet type ID.
### Linking the User ID with a certification self-signature
Similar to [linking a subkey](zoom_enc_subkey) to the OpenPGP certificate, a self-signature is used to connect this new component to the certificate.
To bind identities to a certificate with a self-signature, signature types `0x10` - `0x13` can be used. Here, the signature type `0x13` (*positive certification*) is used.
```text
$ sq packet dump --hex alice.pub-3--Signature
Signature Packet, new CTB, 2 header bytes + 185 bytes
Version: 6
Type: PositiveCertification
Pk algo: Ed25519
Hash algo: SHA512
Hashed area:
Signature creation time: 2023-09-29 15:17:58 UTC (critical)
Key expiration time: P1095DT62781S (critical)
Symmetric algo preferences: AES256, AES128
Hash preferences: SHA512, SHA256
Primary User ID: true (critical)
Key flags: C (critical)
Features: MDC
Issuer Fingerprint: AAA18CBB254685C58358320563FD37B67F3300F9FB0EC457378CD29F102698B3
Unhashed area:
Issuer: AAA18CBB254685C5
Digest prefix: DBB8
Level: 0 (signature over data)
00000000 c2 CTB
00000001 b9 length
00000002 06 version
00000003 13 type
00000004 1b pk_algo
00000005 0a hash_algo
00000006 00 00 00 40 hashed_area_len
0000000a 05 subpacket length
0000000b 82 subpacket tag
0000000c 65 16 ea a6 sig creation time
00000010 05 subpacket length
00000011 89 subpacket tag
00000012 05 a4 8f bd key expiry time
00000016 03 subpacket length
00000017 0b subpacket tag
00000018 09 07 pref sym algos
0000001a 03 subpacket length
0000001b 15 subpacket tag
0000001c 0a 08 pref hash algos
0000001e 02 subpacket length
0000001f 99 subpacket tag
00000020 01 primary user id
00000021 02 subpacket length
00000022 9b subpacket tag
00000023 01 key flags
00000024 02 subpacket length
00000025 1e subpacket tag
00000026 01 features
00000027 22 subpacket length
00000028 21 subpacket tag
00000029 06 version
0000002a aa a1 8c bb 25 46 issuer fp
00000030 85 c5 83 58 32 05 63 fd 37 b6 7f 33 00 f9 fb 0e
00000040 c4 57 37 8c d2 9f 10 26 98 b3
0000004a 00 00 00 0a unhashed_area_len
0000004e 09 subpacket length
0000004f 10 subpacket tag
00000050 aa a1 8c bb 25 46 85 c5 issuer
00000058 db digest_prefix1
00000059 b8 digest_prefix2
0000005a 20 salt_len
0000005b 8a 2d 6f da 67 salt
00000060 35 bc 5d 04 77 b4 9d 67 a8 6e c5 d6 88 53 5f e2
00000070 ef f9 66 08 bf c2 e0 db c0 56 0d
0000007b eb d4 2c a5 19 ed25519_sig
00000080 01 0f ba 26 d0 82 a2 cf 5c eb 7a a9 72 d9 f3 b2
00000090 66 07 8b b2 ba 3d b7 89 e4 76 04 6e 35 24 2b 27
000000a0 29 83 be 91 9c 78 6a cc b4 d5 69 47 76 2c 29 d6
000000b0 54 bf 43 19 04 ff 53 98 c0 d5 0b
```
Because this packet structure closely mirrors the [Direct Key Signature](zooming_in_dks) discussed above, we will cover this succinctly.
We're again looking at a Signature packet. Its `type` is `0x13` ([corresponding to a *positive certification* signature](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-10.html#name-signature-types)).
The designated public key algorithm and hash function for this signature are Ed25519 and SHA512, respectively.
As shown in the text atop this packet dump, the hashed subpacket data contains the following metadata:
- Signature creation time: `2023-09-29 15:17:58 UTC` (**critical**)
- Key expiration time: `P1095DT62781S` (**critical**)
- Symmetric algo preferences: `AES256, AES128`
- Hash preferences: `SHA512, SHA256`
- Primary User ID: `true` (**critical**)
- Key flags: `C` (**critical**)
- Features: `MDC`
- Issuer fingerprint: `AAA18CBB254685C58358320563FD37B67F3300F9FB0EC457378CD29F102698B3`
This is a combination of metadata about the User ID itself (designating this User ID as the *primary User ID* of this certificate), algorithm preferences for this identity, and settings that apply to the primary key.
````{note}
Historically, the self-signature that binds the primary User ID to the certificate also contains subpackets relevant not to the User ID, but to the primary key itself.
Setting key expiration time and key flags on the primary User ID self-signature is one mechanism to configure the primary key.
The interaction between metadata on direct key signatures and User ID binding self-signatures [is subtle](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-11.html#name-notes-on-self-signatures), with changes between version 6 and version 4.
```{admonition} TODO
:class: warning
- link to a section that goes into more depth about "#name-notes-on-self-signatures"?
```
````
This section is followed, again, by the (informational) unhashed subpacket area.
Subsequently, we see a salt value for the signature and the signature itself.
The signature is calculated over a hash. The hash, in this case, is derived from the following data:
- the signature's salt
- the serialized primary key's public data
- the serialized User ID
This section specifies- the serialized self-signature packet (excluding the unhashed area)
Refer to [Computing Signatures](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-11.html#name-computing-signatures) in the RFC for details.
## Certifications (Third Party Signatures)
## Revocations
Reference in a new issue View git blame Copy permalink